1. Field of the Invention
The present invention relates to a Josephson element employing an oxide superconductor operating at below the temperature of liquid nitrogen, and to a process for its production.
2. Description of the Related Art
The creation of Josephson junctions, which are the basis of superconducting electronic devices, involves the use of multiple thin-films, and the superconducting coherence length at their interface is an important parameter. Since all oxide superconductors have low carrier concentrations compared to normal metals, the superconductive characteristics are subtly influenced by the crystalline structure and composition. Therefore, damaged layers are produced by defects at junction interfaces. In such cases, a short coherence length makes it impossible to achieve satisfactory junction characteristics. All superconductors with a high Tc (critical temperature), however, have short coherence lengths. In addition, the quantum phenomenon of the tunneling effect is highly dependent on the geometry of the junction interface. This has meant low reproducibility of the characteristics of Josephson junctions of oxide superconductors. On the other hand, a major advantage of Josephson junctions is low power consumption, and this has been very advantageous for the integration of multiple elements. The homogeneity (reproducibility) of element characteristics thus becomes essential for realizing design performance. In addition, the fact that Josephson junctions have no amplification function necessitates a narrow distribution of element characteristics for realizing circuit performance. This has led to continuous active research for improving the reproducibility of Josephson junctions. One thing that has been learned is that Josephson junctions may be achieved relatively easily by forming plane-configured junctions which allow the a-axis direction, having a relatively long coherence length, to be oriented perpendicular to the junction surface (Japanese Unexamined Patent Publication (Kokai) No. 6-151986).
According to a known process, after preliminary treatment of the (100) surface of a single crystal substrate, such as MgO, SrTiO.sub.3, etc., which may include formation of a groove, amorphization and roughening, a YBCO or BSCCO film is formed, and the treated region is made into a tunneling barrier region. Japanese Unexamined Patent Publication (Kokai) No. 4-152686 discloses formation of a groove 0.2 .mu.m wide and 0.1 .mu.m deep in a MgO (100) single crystal substrate by photo etching (Ar sputtering), followed by formation of a BSCCO film by RF magnetron sputtering to create a tunneling Josephson junction. Japanese Unexamined Patent Publication (Kokai) No. 4-155875 discloses excimer laser irradiation of a MgO or SrTiO.sub.3 (100) single crystal substrate for melting and resolidification to amorphize a 1 .mu.m width, followed by formation of a YBCO or BSCCO film by sputtering. Finally, Japanese Unexamined Patent Publication (Kokai) No. 4-132278 discloses rough finishing of the entire surface of a MgO or SrTiO.sub.3 (100) single crystal substrate with diamond powder, coating an approximately 1 .mu.m width with a resist, and melting of the other sections by Ar ion milling to flatten them, followed by formation of a YBCO film.
With the methods disclosed in the above-mentioned publications, however, it is still difficult to control the critical current value, and it has not been possible to provide sufficient reproducibility for integration. In the case of planar junctions, the barrier layer is roughly the same substance as the thin-film, and the critical current value is highly dependent on the cross-sectional area of the junction. Consequently, it is believed that any irregularities in the quality of the thin-film will lead to variations in the characteristics of the barrier layer or the lateral characteristics during patterning, thus eventually resulting in poor reproducibility. Particularly when a YBCO thin-film is deposited by the pulse laser method, particulates are generated and reside in the junction surface, thus lowering the critical current value.
The present invention relates to a Josephson element which employs a superconductor, and specifically it provides a Josephson element employing a NdBa.sub.2 Cu.sub.3 O.sub.7-y thin-film for the purpose of preventing the generation of particulates by the use of the NdBa.sub.2 Cu.sub.3 O.sub.7-y thin-film which has excellent surface flatness and homogeneity, in order to improve the reproducibility of critical current densities of plane-configured Josephson junctions, thus opening the door to integration of Josephson junctions.